Without limiting the scope of the invention, its background is described in connection with bulk charge modulated device (BCMD) image sensors, as an example- A BCMD photocell is a device used in high performance image sensors for conversion of photon generated charge to an output signal. The photocell is a MOS phototransistor that has a potential well formed under the transistor channel. When the MOS phototransistor is illuminated, photogenerated charges are stored in the potential well. As charge accumulates in this potential well, the transistor threshold is modulated. This modulation is detected by a suitable signal processing circuit. The circuit is sensitive only to the threshold changes caused by light. This is important for elimination of random cell-to-cell fixed pattern noise otherwise induced by the fixed transistor threshold variations.
A typical prior art BCMD photocell has a vertical overflow drain. This device is described in Hynecek, J, "Bulk Charge Modulated Transistor Threshold Image Sensor Elements and Method of Making", U.S. Pat. No. 4,901,129, issued Feb. 13, 1990. The typical prior art device is reset by pulsing the gate voltage so that the charge flows out of the charge well down to the substrate.